Electrical heating properties of various electro-circuit patterns coated on cotton fabric using graphene/polymer composites

Abstract

To investigate the electrical heating performance of two types of electro-circuit patterns, stripe-pattern (SP) and horseshoe-pattern (HP) types were designed by using graphene/poly(vinylidene fluoride-co-hexafluoropropylene) composites to fabricate electrical heating textiles for the inner layers of clothing and gloves to maintain body temperature. To confirm the electrical properties of the pattern shape and area of the coated circuit, the surface resistivity of SP and HP types was measured with various sample lengths, namely 100, 75, and 50 mm, respectively. The surface resistivity of each sample tends to increase linearly with the increasing size of the coated area. In addition, the surface resistivity of the HP is found to be higher than that of the SP. It could be confirmed that the surface resistivity increases as the curvature increases. For the electrical heating properties of the HP, a white-zone and a red-zone appeared clearly, and locally excess heat appeared at the white-zone; the resistive heat can be explained by the collision of the free electrons in the curved shape of the HP area. In order to confirm the applicability of the fabric heating elements, HP100/cotton, HP75/cotton, and HP50/cotton were fabricated by applying the HP to cotton fabric. The difference of surface temperatures at two points of each line of HP100/cotton, HP75/cotton, and HP50/cotton were about 6.0 ± 2.4℃, 6.8 ± 4.5℃, and 3.5 ± 1.7℃, respectively. It has been confirmed that the heating performance is improved, due to the collision of electrons in the curved region with decreasing HP100/cotton to HP50/cotton ratio, and the white-zone is also increased.

Keywords

graphene electro-circuit horseshoe-pattern electrical heating properties graphene printed cotton

Flexible wearable devices based on textiles have started to receive significant interest for applications to wearable smart devices. The “horseshoe-shape” is a stretchable interconnection structure of deformable electronic circuits in E-textiles. It is able to accommodate large deformations in response to mechanical stress, preserving the electrical properties. These are used in common metal conductors due to their limited elastic ranges, and therefore the design of an appropriate shape is crucial to allow flexibility and stretchability of the conductors.^1,2 They are also applied in electric heaters as one of the fabrication methods using a linear conductive wire as a wire-wound heating element. This method can produce a flexible electrical heater using a conductive wire and is electrically stable because the electrical resistance does not change during stretching, but with a large difference between the local temperature maxima and the average working temperature.³ The Drude model is a simple model for handling conductors, assuming that the free electrons in the conductor move between infinitely rigid positive ions.^4,5 In this case, assuming that the electric field is the same inside the conductor, the free electrons accelerate in the opposite direction of the electric field, the accelerated free electrons collide with the cations, and consequently the energy of the ions increases. As the kinetic or vibrational energy of the ions increases through this process, the heat and the temperature of the conductor becomes higher.^6–9

Another main method of manufacturing a flexible conductive textile is coating or printing according to circuit design conditions using conductive ink.^10–15 This method is the most facile and simple and, thus, it is also widely used for producing conductive fabrics, such as electrical heating textiles.^6–9,16,17 Conductive inks are fabricated with conductive nano-materials, such as silver nanowires (AgNWs)¹⁶ and carbon nano-materials,^6–9,17 which are added to polymers. With conductive nano-materials and voltage applied to the composite in which the network structure is formed, the free electrons are randomly disordered without direction as the current flows, and resistance heating appears when they collide.^7,8

Graphene has recently been attracting attention as a conductive filler used for polymers¹⁸; it has strong chemical and heat resistance and excellent mechanical properties and electrical conductivity. Poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) is one of the matrixes used for graphene due to its solubility in organic solvents and formability; research using graphene/PVDF composites is being conducted on piezoelectric materials^19,20 and energy storage devices.²¹ In our previous studies,^22–26 the electrical heating performance of fabric heating elements with graphene/polymer composites were studied. To conduct basic research for developing electric heating textiles, graphene/polymer composite solution and films were prepared and analyzed for their electrical and electrical heating properties. When 50 V was applied to a graphene/polymer composite film containing 8 wt% of graphene, the maximum temperature was 78.7℃, and the possibility of use as a fabric heating element was confirmed.²² Electrical heating textiles were fabricated with graphene/polymer composites coated on cotton and polyester fabric as a planar-type with different coating areas from 10.0 cm × 10.0 cm to 2.5 cm × 2.5 cm.²⁴ The electrical heating property was improved as the coating area was reduced. When the applied voltage was 50 V, the surface temperature of 16 wt% graphene/polymer composite coated on the samples of 10.0 cm × 10.0 cm and 2.5 cm × 2.5 cm area on cotton were 24.5℃ and 52.3℃, respectively, and on polyester fabrics they were 33.2℃ and 78.3℃, respectively. For improving mechanical properties and electrical properties, graphene nanocomposite coated on polyester fabric was annealed from 100℃ to 160℃.²⁵ When annealed at 120℃, the best electrical and electrical heating properties were obtained and mechanical properties were also improved. In order to increase the flexibility, the 8 wt% graphene nanocomposite was coated on para-aramid knit using the dip-coating method. When 50 V was applied, the surface temperatures of the samples that were five-coat coated and hot-pressed at 140℃ were 32.8℃ and 54.8℃, respectively, and the electrical heating performance was increased after the hot-press process.²⁶

Nevertheless, planar-type electrical heating textiles are still rigid and remarkably limited for use in the realization of body-conformable heating devices. Thus, wearable heating elements with various electro-circuit patterns have been developed and studied. In this work we investigate electrical heating performance of two types of electro-circuit patterns; the stripe-pattern (SP) and horseshoe-pattern (HP) types were designed by using graphene/PVDF-HFP composites to fabricate fabric heating textiles. Although research on metal conductors that impart stretchability and flexibility using HPs has been reported,^1,2 there is a lack of research on the electrical and electrical heating properties of samples formed by coating the HP type circuit with nanocomposites. The specific objectives are as follows. Firstly, after designing the circuit in a SP and a HP, the electrical and electric heating properties of the electro-circuit pattern were confirmed and compared. Secondly, to verify the applicability to the fabric heating textiles using the designed electro-circuit patterns, six HP electro-circuits were coated on cotton fabric, which decreased the width and length of the fabric by 25% and 50%, respectively, based on 100 mm × 100 mm samples and then the electric heating performance was analyzed. We have confirmed that two points of temperature range exist in one pattern, and we can confirm the feasibility of applying them to electrical heating elements with two temperature ranges by controlling them.

Experimental details

Materials

The graphene (GNP-UC, Carbon Nano Technology Co. Ltd, Korea) and PVDF-HFP (SOLEF 21508, Solvay Co., Ltd) chips used in this study were the same as those used in our previous study.²³ First-grade acetone (Junsei Chemical Co. Ltd, Japan) was used as the solvent. Flame retardant cotton fabric (0.042 g/cm², twill structure, Mirae Advanced Material Co. Ltd, Korea) was used as the substrate material.

Preparation of the graphene/PVDF-HFP composite solution

To obtain the graphene/PVDF-HFP composite solution, PVDF-HFP chips were dissolved in first-grade acetone by stirring for 1 day at room temperature, and then 15 wt% PVDF-HFP solution was obtained. Later, 16 wt% graphene was added to the 15 wt% PVDF-HFP solution. The prepared graphene/PVDF-HFP composite solutions were stirred for over 1 week with a hotplate stirrer with digital control (MSH-20D, Daihan Scientific, Korea).

Design of electro-circuit patterns

The electrical heating textile was designed with various circuit patterning conditions. It was designed with a SP type (below SP) and HP type (below HP), as shown in Table 1. The width of the electro-circuit patterns was fixed at 2.5 mm, and variation of circuit pattern length were controlled to be reduced by 25%, to 100, 75, and 50 mm, respectively.

Table 1.

Designs of various electro-circuit patterns

Sample code	Type of electro-circuit pattern	Sample length (mm)	Coating area (W × L, mm × mm)
SP100	Stripe-pattern type	100	100 × 2.5
SP75		75	75 × 2.5
SP50		50	50 × 2.5
HP100	Horseshoe-pattern type	100	150 × 2.5
HP75		75	120 × 2.5
HP50		50	90 × 2.5

Fabrication of cotton fabric with various electro-circuit patterns using the graphene/PVDF-HFP composite

For fabricating the electrical heating textile, six electro-circuit patterns of the HP with various circuit pattern lengths were used and coated on sample sizes of 100 mm × 100 mm, 75 mm × 75 mm, and 50 mm × 50 mm, respectively. The electrical heating textiles with various electro-circuit patterns are listed in Table 2. All samples are coated graphene/PVDF-HFP composites with various electro-circuit patterns, and these were obtained by the knife-edge method. The substrate fabric had a thickness of 0.65 mm, and the thickness of the sample after coating was 0.75 mm ± 0.01 mm. Thus, the thickness of the coating was fixed at 0.10 mm ± 0.01 mm. All samples were hot-pressed at 140℃ for 3 min at 3.5 MPa.²⁶ After the hot-press process, the thickness decreased by 33% compared to the un-treated sample.

Table 2.

Design and sample code of electrical heating textiles by horseshoe-pattern (HP) type electro-circuit pattern

Sample code	Type of electro-circuit pattern	Size of samples (mm × mm)
HP100/cotton	Horseshoe-pattern type	100 × 100
HP75/cotton		75 × 75
HP50/cotton		50 × 50

Characterization

The electrical properties of the samples, with various electro-circuit patterns, were analyzed by measuring the surface resistivity with a multimeter (ST850A, Saehan Tester Co. Ltd, Korea) using the AATCC-76 method. Two parallel conductive probes were placed in contact with both edges of the sample. The surface resistivity $(R_{s})$ is expressed in ohm/cm² and was calculated using Equation (1)

R_{s} = (\frac{W}{D}) \times R'

(1)

where

R'

is the resistance measured by the multimeter, W is the width of the sample, and D is the distance between the two electrodes.

The electrical heating properties of the samples, with various electro-circuit patterns, were characterized with a direct current (DC) power supply (CPS-2450B, CHUNGPAEMT. Co. Ltd, Korea). The (+) and (–) poles of the DC power supply were connected by alligator clips to the ends of the sample. The voltage was applied at intervals of 5 V from 0 to 50 V under a thermal imaging camera (FLIR i5, FLIR Systems Inc., USA) and the current was measured at each applied voltage. Three samples were tested and the average was obtained and then this value was used.

To measure the surface temperature of the local area of an electro-circuit pattern, a temperature data logger (TR-71wf, T&D Corp., Korea) and temperature sensor (TR-0206, T&D Corp., Korea) were used. The samples exhibited a surface temperature of 50.0 ± 5.0℃. For the HP100 and HP100/cotton, HP75 and HP75/cotton, and HP50 and HP50/cotton samples, 40, 30, and 20 V, respectively, were applied for 1 h and the temperature values were measured until the temperature reached an equilibrium state. Figure 1 indicates the positions of the test zone.

Figure 1.

The positions of electrical heating test zones of samples: (a) HP100/cotton; (b) HP75/cotton; (c) HP50/cotton. HP: horseshoe-pattern.

Results and discussion

Surface resistivity of various electro-circuit patterns designed as SP and HP the using graphene/PVDF-HFP composite

The surface resistivity of various electro-circuit patterns is indicated in Figure 2. In this study, SP and HP electro-circuit patterns were designed and the surface resistivity was measured in order to investigate the pattern shape and the area of the coated circuit. As shown in Figure 2, the surface resistivity of the SP and HP tends to increase linearly with increasing coating size: R² of the SP and HP is $0.9999$ and $0.9716$ , respectively. The surface resistivity of samples SP100, SP75, and SP50 were 7.3 × 10⁴ ± 2.3 × 10⁴, 2.8 × 10⁴ ± 1.2 × 10³, and 1.0 × 10⁴ ± 3.0 × 10³ Ω/cm², respectively, and HP100, HP75, and HP50 indicated 8.8 × 10⁴ ± 1.4 × 10⁴, 4.6 × 10⁴ ± 7.6 × 10³, and 2.3 × 10⁴ ± 2.2 × 10³ Ω/cm², respectively. Filipowska et al.¹⁵ reported that when the electro-conductive paths and patterns were applied to flat textiles with a screen printing method, the line width increased, the paths and patterns of electro-conductivity increased, the line length decreased, and the paths and patterns of electro-conductivity decreased. In addition, it is generally confirmed that the resistance increases as the length increases, because the resistance (R) is proportional to the length (L) and inversely proportional to the cross-section area (A) in the formula of $R \propto \frac{L}{A}$ .²⁴

Figure 2.

Surface resistivity of various electro-circuit patterns designed as stripe-pattern (SP) and horseshoe-pattern (HP) types with the graphene/poly(vinylidene fluoride-co-hexafluoropropylene) composite by coated area.

In addition, when the samples coated with the same content of graphene composite were used, the surface resistivity of the HP was higher than that of the SP. Generally, an electrical heater fabricated with a metal wire of wire-wound type like a HP is electrically stable because the electrical resistance does not vary during stretching. However, a large difference appeared between the local temperature maxima and the average working temperature.³ In this case, a high stress and strain concentration is observed in the crest and trough of the line, like a curved-shape area of HP samples. In the inner radius, compressive stresses are present, while in the outer radius, tensile stresses are observed.² The morphology is related to the electrical properties, and it is confirmed that when the electro-circuit line width is constant and the length of the inner part of the HP increases more than the SP, the surface resistivity tends to increase as the area of the inner part of the HP increases more than SP.

Electrical heating properties of various electro-circuit patterns designed as SP and HP types with the graphene/PVDF-HFP composite

To investigate the electrical heating properties of various electro-circuit patterns designed as SP and HP types with the graphene/PVDF-HFP composite, infrared (IR) thermal images of the surface temperatures of the samples are shown in Tables 3–5. The surface temperature of each sample was measured after various voltages from 5 to 50 V with intervals of 5 V were applied for 3 min, and the images in the tables are shown at intervals of 10 V.

Table 3.

Infrared thermal image of surface temperature and current for electro-circuit patterns designed as SP100 and HP100 using the graphene/poly(vinylidene fluoride-co-hexafluoropropylene) composite at various applied voltages

Applied voltage (V)		Sample code
Applied voltage (V)		SP100	HP100
Control	Surface image
	Mean temp. (℃)	20.5 ± 0.2	20.5 ± 0.2
	Max temp. (℃)	24.0 ± 0.2	24.0 ± 0.2
	Current (A)	0.00	0.00
10	Surface image
	Mean temp. (℃)	23.4 ± 0.7	22.2 ± 1.0
	Max temp. (℃)	25.7 ± 1.2	26.0 ± 2.7
	Current (A)	0.01	0.01
20	Surface image
	Mean temp. (℃)	29.1 ± 2.0	26.7 ± 1.8
	Max temp. (℃)	31.0 ± 0.0	33.0 ± 2.7
	Current (A)	0.01	0.01
30	Surface image
	Mean temp. (℃)	38.4 ± 3.6	35.5 ± 1.3
	Max temp. (℃)	41.0 ± 1.4	47.0 ± 1.7
	Current (A)	0.02	0.02
40	Surface image
	Mean temp. (℃)	50.2 ± 4.5	46.5 ± 2.4
	Max temp. (℃)	50.5 ± 0.7	60.6 ± 3.5
	Current (A)	0.03	0.03
50	Surface image
	Mean temp. (℃)	67.5 ± 5.9	59.4 ± 2.8
	Max temp. (℃)	68.0 ± 4.3	71.3 ± 2.1
	Current (A)	0.04	0.03

SP: stripe-pattern; HP: horseshoe-pattern

Table 5.

Infrared thermal image of surface temperature and current for electro-circuit patterns designed as SP50 and HP50 using the graphene/poly(vinylidene fluoride-co-hexafluoropropylene) composite at various applied voltages

Applied voltage (V)		Sample code
Applied voltage (V)		SP50	HP50
Control	Surface image
	Mean temp. (℃)	20.5 ± 0.2	20.5 ± 0.2
	Max temp. (℃)	24.0 ± 0.2	24.0 ± 0.2
	Current (A)	0.00	0.00
10	Surface image
	Mean temp. (℃)	29.4 ± 1.4	28.9 ± 0.9
	Max temp. (℃)	34.3 ± 4.2	31.7 ± 0.6
	Current (A)	0.02	0.02
20	Surface image
	Mean temp. (℃)	58.2 ± 3.4	52.3 ± 3.2
	Max temp. (℃)	77.7 ± 4.2	79.3 ± 7.2
	Current (A)	0.04	0.03
30	Surface image
	Mean temp. (℃)	103.3 ± 5.1	93.7 ± 5.5
	Max temp. (℃)	123.7 ± 9.1	125.0 ± 10.5
	Current (A)	0.05	0.05
40	Surface image
	Mean temp. (℃)	146.0 ± 9.5	145.3 ± 9.5
	Max temp. (℃)	149.7 ± 7.5	155.3 ± 6.4
	Current (A)	0.08	0.06
50	Surface image	Burning	Burning
	Mean temp. (℃)	–	–
	Max temp. (℃)	–	–
	Current (A)	–	–

SP: stripe-pattern; HP: horseshoe-pattern

As shown in Table 3, IR thermal images of surface temperatures and current values of SP100 and HP100 were obtained at various applied voltages. In both types of SP100 and HP100, the current value and the surface temperature of the sample were increased as the applied voltage increased according to Joule's law.^6–9 When 50 V was applied to each sample, the current value and surface temperature of SP100 tended to be higher than that of HP100. The current value and surface temperature of SP100 were 0.04 A and 67.5 ± 5.2℃, respectively, and for HP100 they were 0.03 A and 59.4 ± 2.8℃. As shown in the IR image, the HP electro-circuit pattern indicated two-point surface temperature zones as a white-zone and a red-zone. The white-zone in the IR thermal image indicated the highest temperature of the whole heating area, and the red-zone is the average temperature of the samples. It can be confirmed that SP100 generates heat when voltage is applied. However, in the case of HP100, a white-zone is formed in the region having a curved shape, which seems to generate the excess heat at the curved region.³

The samples that showed a temperature of about 50℃, which is a suitable temperature for a fabric heating element, were selected and this occurred when 40 V was applied. When 40 V is applied, the average surface temperature of the red-zone is indicated 46.5 ± 2.4℃, and that of the white-zone, the highest surface temperature, is about 60.0℃. Figure 4 shows the temperature difference between the white-zone and the red-zone.

Table 4 presents IR thermal images and currents of SP75 and HP75 at various applied voltages. As for the previous IR thermal images of SP100 and HP100, the current values and the surface temperatures of SP75 were higher than those of HP75 when 50 V was applied and the currents were 0.06 A and 0.05 A, respectively, and the heating temperatures were 123.5 ± 6.4℃ and 98.2 ± 2.8℃, respectively.

Table 4.

Infrared thermal image of surface temperature and current for electro-circuit patterns designed as SP75 and HP75 using the graphene/poly(vinylidene fluoride-co-hexafluoropropylene) composite at various applied voltages

Applied voltage (V)		Sample code
Applied voltage (V)		SP75	HP75
Control	Surface image
	Mean temp. (℃)	20.5 ± 0.2	20.5 ± 0.2
	Max temp. (℃)	24.0 ± 0.2	24.0 ± 0.2
	Current (A)	0.00	0.00
10	Surface image
	Mean temp. (℃)	25.7 ± 0.8	24.2 ± 0.6
	Max temp. (℃)	30.7 ± 4.0	29.7 ± 0.6
	Current (A)	0.01	0.01
20	Surface image
	Mean temp. (℃)	41.7 ± 4.9	35.4 ± 0.9
	Max temp. (℃)	51.3 ± 8.9	41.3 ± 2.8
	Current (A)	0.03	0.02
30	Surface image
	Mean temp. (℃)	68.2 ± 8.7	52.9 ± 1.6
	Max temp. (℃)	76.5 ± 5.3	58.7 ± 1.2
	Current (A)	0.04	0.03
40	Surface image
	Mean temp. (℃)	96.6 ± 2.0	74.7 ± 1.2
	Max temp. (℃)	116.5 ± 8.2	85.0 ± 1.0
	Current (A)	0.05	0.04
50	Surface image
	Mean temp. (℃)	123.5 ± 6.4	98.2 ± 2.8
	Max temp. (℃)	133.6 ± 9.0	126.7 ± 1.5
	Current (A)	0.06	0.05

SP: stripe-pattern; HP: horseshoe-pattern.

When the sample length was decreased from 100 to 75 mm by 25%, the current value of SP75 and HP75 increased by 0.02 A when 50 V was applied and the surface temperature increased by about 50℃ and 40℃, respectively. It can be seen that when the applied voltage is the same, the electric heating performance is improved because the heat generating area is reduced.^4,7 Also, as mentioned above for SP100, when voltage is applied to SP75, the IR image shows that heat is generated. In the case of HP75, the surface temperature in the curved region is higher than that in the non-curved region; thus, the curved and non-curved regions show a white-zone and a red-zone, respectively.

Table 5 shows IR thermal images and currents of SP50 and HP50 at various applied voltages. For SP50 and HP50 with 25 mm length reduction compared to SP75 and HP75, the current and surface temperatures were high, and both samples showed a surface temperature over 50℃ when 20 V was applied. Thus, it is confirmed that the electrical performance is much improved. When the voltage of 50 V was applied, the current value of the two samples was suddenly supplied, and the surface temperature was dramatically increased. As a result, the samples began burning and were damaged, and were measured up to 40 V. For SP50 and HP50, the current values were 0.08 and 0.06 A when 40 V was applied, and the surface temperatures were 146.0 ± 9.5℃ and 145.3 ± 9.5℃, respectively. The difference in current between SP50 and HP50 was 0.02 A, but the surface temperature had similar values. In the IR thermal image of HP50, the white-zone of the curved-shape region appears as dots, and it is clearly different from HP100 and HP75. The HP with curves has been reported to be characterized by a high stress and strain concentration in the inner radius.² As the sample size of the HP decreases, the size and height of the amplitude are reduced and the inner radius of the curved shape is seen to be smaller. Therefore, it is confirmed that the white-zone appears in the form of a dot in this region, and the white-zone seems to be connected, so it appears to represent a stripe shape.

As shown in the Tables 3–5, it could be seen that the electric heating performance is improved with decreasing length of SP and HP samples. Previous studies^7,24 reported fabric heating elements based on nanocomposites with different sample dimensions. The smaller the dimension of the samples, the better the heating performance. Generally, the total power consumption (P) can be calculated using Equation (2)

P = \frac{V^{2}}{R}

(2)

where P denotes the total power consumption, R the resistance, and V is the applied voltage. As the sample dimension decreased, the resistivity decreased and a higher steady-state temperature was obtained. The small surface area helps to dissipate the heat rapidly and it could be lead to a higher steady-state temperature, where the heater loses its heat to the air by convection and to the cotton by conduction.

Figure 3 shows the relationship between the applied voltage and the average heating temperature of the SP and the HP samples for different sample lengths. As shown in Tables 3–5, the SP and HP show that the surface temperature rises as the applied voltage increases, regardless of sample length and the shape of the pattern. In addition, it can be seen that as the length of the sample decreases, the electrical heating performance is improved. It is confirmed that when the same applied voltage is applied, the electrical heating performance is improved, while reducing the heating area.⁷

Figure 3.

Changes of electrical heating temperature of stripe-pattern (SP) and horseshoe-pattern (HP) types with various sample lengths and applied voltages.

As seen in the IR image, a temperature difference at the two-point zone was observed; Figure 4 shows the surface temperature graph of the white-zone (the highest heating temperature) and the red-zone (average heating temperature) of the HP in order to confirm the difference for the samples with the curved shape. Samples exhibiting a surface temperature of 50.0 ± 5.0℃ for use as an electrical heating element for clothing were used in the test. For the HP100, HP75, and HP50 samples, 40, 30, and 20 V were applied, respectively, for 1 h and the temperature values were measured until the temperature reached equilibrium. The inserted IR images in Figure 4 indicate the HP50 samples; those of the highest temperature had 20 V applied for 300, 2400, and 3600 s. The surface temperature of the curved shape area of the HP was higher than for the non-curved shape area. The difference of surface temperature between the red-zone and white-zone for both HP100 and HP75 was about 6.6 ± 0.8℃ and 6.5 ± 0.6℃, respectively, and for HP50 it was about 1.7 ± 0.5℃.

Figure 4.

Electrical heating behavior of the red-zone (average heating area) and the white-zone (the highest heating area) of the horseshoe-pattern (HP). Black: HP100, direct current (DC) voltages 40 V; blue: HP75, DC voltage 30 V; red: HP50, DC voltage 20 V (color online only).

A schematic of the electron transport mechanism of various electro-circuit patterns is shown in Figure 5. For the electrical heating textiles composed of conductive nano-materials, when voltage is applied to the composite in which the network structure is formed, the free electrons are randomly disordered and without direction as the current flows, and they appear as a resistance heating when they collide.⁸ In general, graphene has a very high specific surface area. When the graphene added in a polymer, large numbers of interfaces are produced and these will lead to phonon scattering and introduce ultrahigh interfacial thermal resistance.⁵ The heating mechanism using by carbon-based materials is, initially, the migration of charge carriers in the system that may have been accelerated due to the application of an external electric potential. These accelerated electrons might collide in-elastically with phonons, impurities, or defects and release the heat through collision. The mean free path available for the electrons starts to reduce on increasing the applied potentials, which results in increased scattering effects that are responsible for an observed rise in temperature.^7,27 When voltage is applied to the electro-circuit patterns, the current flows from (+) to (–). As for comparing a HP sample with a SP sample with the same overall dimensions, the HP sample has a longer resistive path and larger coated area than the SP sample. The mean temperature indicated in the red-zone is lower than for the SP sample. However, the curved shape of HP samples indicates a compressive stress on the inner radius in the morphology, resulting in a decrease in the coating area compared to the straight-shape, and it is confirmed that more heat is generated. In this study, it was confirmed that a pattern has a two-point temperature range and it can be applied as a heating element with two temperature ranges by controlling it, and also the electrical heating properties can be improved by reducing the size of the pattern.

Figure 5.

Schematic of the electron transport mechanism of various electro-circuit patterns designed as stripe-pattern and horseshoe-pattern types. PVDF-HFP: poly(vinylidene fluoride-co-hexafluoropropylene).

Electrical heating properties of cotton fabric coated with the HP using the graphene/PVDF-HFP composite

In order to confirm the possibility of using the HP manufactured in this study as a fabric heating element, samples were prepared with the width and length of the fabric set to 100 mm, which was reduced to 25% and 50%, 75 mm × 75mm, and 50 mm × 50 mm, respectively. Table 6 shows the IR thermal images and currents of the electrical heating textiles of HP100/cotton, HP75/cotton, and HP50/cotton with six voltage values. Figure 6 shows the change in electrical heating temperature of HP100/cotton, HP75/cotton, and HP50/cotton with applied voltage. HP100/cotton, HP75/cotton, and HP50/cotton samples were found to increase the current value by about three times as compared to HP100, HP75, and HP50, which seems to be due to an increase in the number of electro-circuits.

Figure 6.

Changes of electrical heating temperature of HP100/cotton, HP 75/cotton, and HP50/cotton with applied voltages. HP: horseshoe-pattern.

Table 6.

Infrared thermal image of temperature and current for the electro-circuit pattern of the horseshoe-pattern (HP) coated on cotton fabric using the graphene/poly(vinylidene fluoride-co-hexafluoropropylene) composite at various applied voltages

Applied voltage (V)		Sample code
Applied voltage (V)		HP100/cotton	HP75/cotton	HP50/cotton
Control	Surface image
	Mean temp. (℃)	20.5 ± 0.2	20.5 ± 0.2	20.5 ± 0.2
	Max temp. (℃)	24.0 ± 0.2	24.0 ± 0.2	24.0 ± 0.2
	Current (A)	0.00	0.00	0.00
10	Surface image
	Mean temp. (℃)	24.2 ± 0.7	23.9 ± 0.1	29.4 ± 1.3
	Max temp. (℃)	28.3 ± 0.6	26.7 ± 0.6	29.7 ± 2.1
	Current (A)	0.02	0.03	0.07
20	Surface image
	Mean temp. (℃)	29.5 ± 0.7	30.6 ± 0.4	49.3 ± 4.5
	Max temp. (℃)	31.7 ± 1.2	35.7 ± 1.2	53.0 ± 4.2
	Current (A)	0.03	0.07	0.14
30	Surface image
	Mean temp. (℃)	36.3 ± 1.3	43.2 ± 1.5	72.8 ± 3.2
	Max temp. (℃)	41.3 ± 2.5	53.7 ± 4.6	80.3 ± 5.5
	Current (A)	0.05	0.11	0.21
40	Surface image
	Mean temp. (℃)	49.8 ± 3.3	65.5 ± 6.7	101.0 ± 9.5
	Max temp. (℃)	54.3 ± 5.9	74.7 ± 5.5	124.7 ± 1.2
	Current (A)	0.07	0.15	0.24
50	Surface image			Burning
	Mean temp. (℃)	64.4 ± 5.4	71.2 ± 2.7	-
	Max temp. (℃)	68.0 ± 6.9	96.7 ± 8.2	-
	Current (A)	0.09	0.18	-

As shown in the previous electrical heating performance of electro-circuit patterns, the heating performance tended to increase as the length and area of the sample decreased. In the case of HP50/cotton, the sample showed burning at 45 V and it should be measured to 40 V. As 40 V was applied to the HP50/cotton, the surface temperature increased to 101.0 ± 9.5℃. As shown in the IR thermal image, when the fabric is an electrical heating textile, the fabric heating element shows heat generated over the entire area as the area is decreased. In addition, it has been confirmed that the heating performance is improved due to the collision of electrons in the curved-shape region with decreasing sample size from HP100 to HP50. This shows that the white-zone is increased compared to HP100/cotton and HP75/cotton, as shown in the IR thermal image of HP50/cotton, and the heating area is also improved.

The difference between the white-zone and the red-zone of the curved shape in each line of the electrical heating textile is shown in Figures 7–9. HP100/cotton, HP75/cotton, and HP50/cotton exhibited about 50.0 ± 5.0℃ at applied voltages of 40, 30, and 20 V, respectively. Thus, those samples were selected and tested. As shown in Figure 7, the overall temperature range for the HP100/cotton sample between the red-zone and white-zone varied from about 45.0℃ to 55.0℃. The surface temperature range of the red-zone indicates under 50.0℃ and the white-zone shows generally over 50.0℃. The difference in surface temperature at two points of each line of the HP100/cotton sample was about 6.0 ± 2.4℃. Figure 8 shows that the overall temperature range for the HP75/cotton sample between the red-zone and white-zone is about 45.0–60.0℃; it is higher than for the HP100/cotton sample, and the difference in surface temperature at two points of each line of the HP75/cotton sample is about 6.8 ± 4.5℃. The surface temperature of the red-zone of the HP75/cotton sample is lower than about 53.0℃ and that of the white-zone is higher than about 53.0℃. In the case of the HP50/cotton sample, the overall temperature range between the red-zone and white-zone is about 48.0–65.0℃, as shown in Figure 9. The difference in surface temperature at two points of each line of the HP50/cotton sample is about 3.5 ± 1.7℃, which is the lowest of the samples, but the surface temperature between the red-zone and white-zone of each line could be distinguished.

Figure 7.

Electrical heating behavior of the red-zone (solid line) and the white-zone (dashed line) of the curved-shape area of HP100/cotton samples at applied voltages of 40 V. HP: horseshoe-pattern.

Figure 8.

Electrical heating behavior of the red-zone (solid line) and the white-zone (dashed line) of the curved-shape area of HP75/cotton samples at applied voltages of 30 V. HP: horseshoe-pattern.

Figure 9.

Electrical heating behavior of the red-zone (solid line) and the white-zone (dashed line) of the curved-shape area of HP50/cotton samples at applied voltages of 20 V. HP: horseshoe-pattern.

In case of the HP100/cotton when the voltage is applied, the time–temperature curve of the white-zone initially rises sharply, the curve of the red-zone appears smooth, and the temperature is maintained. Then the temperature of the white-zone is gradually lowered, while the temperature of the red-zone is raised to show a graph like a bottleneck. This is because the electron movement is interfered with and the electrons collide, so that the initial temperature of the white-zone is higher and the red-zone shows a relatively lower temperature. Also, as the voltage is continuously applied, the electrons are moved gradually and, as a result, the heating temperature of the white-zone is lowered, and the red-zone has a relatively higher heating temperature. For the HP50/cotton, the size of the electric heating textile is reduced and the width of the HP electro-circuit lines is decreased with the application of the HP50 as six lines in cotton fabric. The electrical heating performance of the HP50/cotton is exhibited throughout the fabric as in the IR image. By using and controlling the temperature difference and heating performance by varying sample sizes, it seems to be possible to apply the electrical heating textiles as heating elements in clothes and gloves to maintain body temperature.

Conclusion

In this study, an electro-circuit pattern with SP and HP types was designed to evaluate the electrical and thermal performance variation with pattern shape, and to verify the feasibility of applying electric heating textiles to the inner layer of clothing and gloves to maintain body temperature. The HP with a two-point temperature range was applied to cotton fabrics of various sample sizes to fabricate the heating elements.

To confirm the electrical properties of the pattern shape and area of the coated circuit, the surface resistivity of the SP and HP was measured for various sample lengths, namely 100, 75, and 50 mm. The surface resistivity of each sample tends to increase linearly with increasing coating area size, and the surface resistivity of the HP was shown to be higher than that of the SP. It was confirmed that the curved-shape region of the HP seems to hinder the flow of free electrons, and then the free electrons cannot easily move and appear to have increased the resistance due to collisions. This could be confirmed by electrical heating properties. In the HP, a white-zone and a red-zone appeared clearly, and locally excess heat appeared at the white-zone. This could be explained as resistive heat due to the collision of the free electrons in the HP in the curved-shape area. The difference between the red-zone and the white-zone of HP100, HP75, and HP50 was 6.6 ± 0.8℃, 6.5 ± 0.6℃, and 1.7 ± 0.5℃, respectively. In order to confirm the applicability of the fabric heating elements, HP100/cotton, HP75/cotton, and HP50/cotton samples were fabricated by applying the HP to cotton fabric. It has been confirmed that the heating performance is improved due to the collision of electrons in the curved-shape region with decreasing HP100/cotton to HP50/cotton and the white-zone is also increased. Also, as the size of the samples decreases, the electrical heating performance improves and the overall heat generation area tends to increase. To investigate the two-point temperature range, the samples that show 50.0 ± 5.0℃ surface temperature were selected, and voltages of 40, 30, and 20 V were applied to HP100/cotton, HP75/cotton, and HP50/cotton samples, respectively. All the samples indicated similar temperature distributions from about 45.0℃ to 60.0℃. Also, the difference in surface temperature at the two-point zone of each line of HP100/cotton, HP75/cotton, and HP50/cotton was about 6.0 ± 2.4℃, 6.8 ± 4.5℃, and 3.5 ± 1.7℃, respectively.

Therefore, this study confirmed that the electric heating textiles of the HP coated on cotton fabric could be manufactured and the electric heating properties, according to the pattern type, were confirmed. It was confirmed that the temperature range of two regions in one pattern can be controlled and it can be applied to a fabric heating element having two temperatures. In addition, by using and controlling those temperature differences and the heating performance for various sample sizes, it seems to be possible to apply an electrical heating textile as a heating element in clothes and gloves to maintain body temperature.

Footnotes

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This research was supported by the Basic Science Research Program through the National Research of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (Grant Number NRF-2016M3A7B4910552).

References

Stoppa

Chiolerio

. Wearable electronics and smart textiles: a critical review. Sensors 2014; 14: 11957–11992.

Gonzalez

Axisa

Bulcke

, et al. Design of metal interconnects for stretchable electronic circuits. Microelectron Reliab 2008; 48: 825–832.

Lee

Kim

J-G

, et al. Versatile, high-power, flexible, stretchable carbon nanotube sheet heating elements tolerant to mechanical damage and severe deformation. Adv Funct Mater 2018; 28: 1706007.

Wikipedia. Drude model, https://en.wikipedia.org/wiki/Drude_model (accessed 8 July 2018).

Zhang

Y-F

. Thermal conductivity of graphene-polymer composites: mechanisms, properties, and applications. Polymers 2017; 9: 437.

Markevicius

Furferi

Olsson

, et al. Towards the development of a novel CNTs-based flexible mild heater for art conservation. Nanomater Nanotechnol 2014; 4: 8.

Ilanchezhiyan

Zakirov

Kumar

, et al. Highly efficient CNT functionalized cotton fabrics for flexible/wearable heating applications. RSC Adv 2015; 5: 10697–10702.

Kang

Lee

. Preparation and characterization of carbon nanofiber composite coated fabric-heating elements. J Korean Soc Cloth Text 2015; 39: 247–256. .

Kim

Lee

. Characterization of carbon nanofiber (CNF)/polymer composite coated on cotton fabrics prepared with various circuit patterns. Fash Text 2018; 5: 7.

10.

Locher

Tröster

. Screen-printed textile transmission lines. Text Res J 2007; 77: 837–842.

11.

Kim

Yoo

H-J

. Electrical characterization of screen-printed circuits on the fabric. IEEE Trans Adv Package 2010; 33: 196–205.

12.

Kwon

O-S

Kim

, et al. Fabrication and characterization of inkjet-printed carbon nanotube electrode patterns on paper. Carbon 2013; 58: 116–127.

13.

Chen

Zhang

, et al. Fabrication of nanoscale circuits on inkjet-printing patterned substrates. Adv Mater 2015; 27: 3928–3933.

14.

Stempien

Rybicki

, et al. Inkjet-printing deposition of silver electro-conductive layers on textile substrates at low sintering temperature by using an aqueous silver ions-containing ink for textronic applications. Sensors Actuators B 2016; 224: 714–725.

15.

Filipowska

Wiśniewski

Michalak

. Creation of electro-conductive paths and patterns by screen printing on textile bases. Text Res J 2018; 88: 261–274.

16.

Kim

G-L

Lee

J-J

Y-J

, et al. Smart wearable heaters with high durability, flexibility, water-repellent and shape memory characteristics. Comp Sci Technol 2017; 152: 173–180.

17.

Lee

J-G

Lee

J-H

, et al. Highly flexible, stretchable, wearable, patternable and transparent heaters on complex 3D surfaces formed from supersonically sprayed silver nanowires. J Mater Chem A 2017; 5: 6677–6685.

18.

Potts

Dreyer

Bielawski

, et al. Graphene-based polymer nanocomposites. Polymer 2011; 52: 5–25.

19.

Kumar

Gau

Rai

, et al. Piezo devices using poly(vinylidene fluoride)/reduced graphene oxide hybrid for energy harvesting. Nano Struct Nano Obj 2017; 12: 174–181.

20.

Gao

, et al. Ultrathin multifunctional graphene-PVDF layers for multidimensional touch interactivity for flexible displays. ACS Appl Mater Interfaces 2017; 9: 18410–18416.

21.

Abolhasani

Shirvanimoghaddam

Naebe

. PVDF/graphene composite nanofibers with enhanced piezoelectric performance for development of robust nanogenerators. Comp Sci Technol 2017; 138: 49–56.

22.

Kim

Lee

. Electrical properties of graphene/waterborne polyurethane composite films. Fiber Polym 2017; 17: 1304–1313.

23.

Kim

Lee

. Fine structure and physical properties of graphene/poly(vinylidene fluoride-co-hexafluoropropylene) composite films prepared under various drawing conditions. Text Sci Eng 2017; 54: 109–115.

24.

Kim

Lee

. Effect on the electrical heating of textiles-coated graphene/waterborne polyurethane composites with different coating areas. Text Sci Eng 2017; 54: 315–326.

25.

Kim

Lee

. Characteristics of electrical heating elements coated with graphene nanocomposite on polyester fabric: effect of different graphene contents and annealing temperatures. Fiber Polym 2017; 19: 965–976.

26.

Kim H, Lee S and Kim H. Electrical properties of dip-coated aramid knit manufactured with graphene/waterborne polyurethane (WPU) composite. In: proceedings of the 2017 Kyushu-seibu/Pusan-Gyeongnam joint symposium (ed. K Sakurai), Kitakyushu, Japan, 14–16 December 2017, paper no. 8, pp.87. Gagoshima: Pusan-Kyeongnam branch of the Korean Fiber Society.

27.

Sarma

Adam

Hwang

, et al. Electronic transport in two dimensional graphene. Rev Mod Phys 2011; 83: 407–470.